1. Field of the Invention
The present invention relates generally to electrospinning of fibers and more particularly to controlled electrospinning of fibers.
2. Background Art
Electrospinning has been known, since the 1930's. However, electrospinning of fibers has not previously gained significant industrial importance, owing to a variety of issues, some of these having been low output, inconsistent and low molecular orientation, poor mechanical properties, difficulties and instabilities of fluid streams in forming fibers, and high diameter distribution of the electrospun fibers. Although special needs of military, medical and filtration applications have stimulated recent studies and renewed interest in the electrospinning, quantitative technical and scientific information regarding process and product characterization are extremely limited.
In a typical electrospinning system, a charged polymer solution (or melt) is fed through a small opening or orifice of a nozzle (usually a needle or pipette tip), and because of its charge, the polymer solution is drawn (as a jet) toward a collector, which is often a grounded collecting plate (usually a metal screen, plate, or rotating mandrel), typically 5-30 cm from the orifice of the nozzle. During the jet's travel, the solvent gradually evaporates, and a charged polymer fiber is left to accumulate on the grounded target. The charge on the fibers eventually dissipates into the surrounding environment. The resulting product is a non-woven fiber mat that is composed of tiny fibers with diameters between 50 nanometers and 10 microns. This non-woven mat forms the foundation of a “scaffold”. If the target is allowed to move with respect to the nozzle position, specific fiber orientations (parallel alignment or a random) can be achieved. Previous work has shown that varying the fiber diameter and orientation can vary the mechanical properties of the scaffold.
Using electrical forces alone, electrospinning can produce fibers with nanometer diameters. Electrospun fibers have large surface to volume ratios, because of their small diameters, which enable them to absorb more liquids than do fibers having large diameters, and small pore sizes make them suitable candidates for military and civilian filtration applications. It is expected that electrospun fibers will find many applications in composite materials and as reinforcements.
Typically, an electric field is used to draw a positively charged polymer solution from an orifice of a nozzle to a collector, and “electrospin” the polymer solution, as the polymer solution travels from the orifice to the collector. A jet of solution typically flows or travels from the orifice of the nozzle to the collector, which is typically grounded. The jet emerges from the nozzle, which is typically of a conical geometry, and often, in particular, a Taylor cone. The jet transitions to form a stretched jet, after the jet leaves the orifice of the nozzle, and then the jet divides into many fibers in an area called the “splaying region”.
As the ionized jet of positively charged polymer solution travels from the orifice to the collector, a “whipping motion” (or bending instability) results in the jet
There is thus a need for apparatus and methods that control the jet and minimize instabilities of the jet as it travels from the nozzle to the collector plate. The apparatus and methods should be capable of controlling the jet, the path of the jet, controlling and minimizing instabilities of these fluid streams during formation of fibers, and controlling the direction of the jet and concentration of solution during electrospinning.
The formation of fibers by electrospinning is also impacted by the viscosity of spinnable fluids, since some spinnable fluids are so viscous that they require higher forces than electric fields can typically produce without arcing, i.e., dielectric breakdown of the air. Likewise, these techniques have been problematic where high temperatures are required, since high temperatures typically increase the conductivity of structural parts and complicate the control of high strength electrical fields. The apparatus and methods should, thus, also be capable of controlling the jet and minimizing instabilities for fluids of different viscosities, and should be capable of controlling the jet during the use of extreme temperatures and high strength electrical fields.
The apparatus and methods that control and minimize instabilities of the jet should be capable of improving efficiency, productivity, and economy of the electrospinning process. The apparatus and methods should also be capable of more accurate use of fluids, improvements in production and formation of fibers, and improvements in the production rate, fiber diameter distribution, measure, and characterization of the electrospun fiber properties in terms of size, orientation and mechanical properties.
Different electrospinning apparatus and methods have heretofore been known. However, none of the electrospinning apparatus and methods adequately satisfies these aforementioned needs.                U.S. Pat. No. 6,713,011 (Chu, et al.) discloses an apparatus and method for electrospinning polymer fibers and membranes. The method includes electrospinning a polymer fiber from a conducting fluid in the presence of a first electric field established between a conducting fluid introduction device and a ground source and modifying the first electric field with a second electric field to form a jet stream of the conducting fluid. The method also includes electrically controlling the flow characteristics of the jet stream, forming a plurality of electrospinning jet streams and independently controlling the flow characteristics of at least one of the jet streams. The apparatus for electrospinning includes a conducting fluid introduction device containing a plurality of electrospinning spinnerets, a ground member positioned adjacent to the spinnerets, a support member disposed between the spinnerets and the ground member and movable to receive fibers formed from the conducting fluid, and a component for controlling the flow characteristics of conducting fluid from at least one spinneret independently from another spinneret. U.S. Pat. No. 4,689,186 (Bornat) discloses production of electrostatically spun products, comprising electrostatically spinning a fiberizable liquid, the electrostatic field being distorted by the presence of an auxiliary electrode, preferably so as to encourage the deposition of circumferential fibers, having tubular portions.        U.S. Pat. No. 6,520,425 (Reneker) discloses a process and apparatus for the production of nanofibers, in which a nozzle is used for forming nanofibers by using a pressurized gas stream comprises a center tube, a first supply tube that is positioned concentrically around and apart from the center tube, a middle gas tube positioned concentrically around and apart from the first supply tube, and a second supply tube positioned concentrically around and apart from the middle gas tube. The center tube and first supply tube form a first annular column. The middle gas tube and the first supply tube form a second annular column. The middle gas tube and second supply tube form a third annular column. The tubes are positioned, so that first and second gas jet spaces are created between the lower ends of the center tube and first supply tube, and the middle gas tube and second supply tube, respectively. A method for forming nanofibers from a single nozzle is also disclosed.        U.S. Pat. No. 6,641,773 (Kleinmeyer, et al.) discloses electro spinning of submicron diameter polymer filaments, in which an electro spinning process yields substantially uniform, nanometer diameter polymer filaments. A thread-forming polymer is extruded through an anodically biased die orifice and drawn through an anodically biased electrostatic field. A continuous polymer filament is collected on a grounded collector. The polymer filament is linearly oriented and uniform in quality. The filament is particularly useful for weaving body armor, for chemical/biological protective clothing, as a biomedical tissue growth support, for fabricating micro sieves and for microelectronics fabrication.        U.S. Pat. No. 6,991,702 (Kim) discloses an electrospinning apparatus, including a spinning dope main tank, a metering pump, a nozzle block, a collector positioned at the lower end of the nozzle block for collecting spun fibers, a voltage generator, a plurality of units for transmitting a voltage generated by the voltage generator to the nozzle block and the collector, the electrospinning apparatus containing a spinning dope drop device positioned between the metering pump and the nozzle block the spinning dope drop device having (i) a sealed cylindrical shape, (ii) a spinning dope inducing tube and a gas inletting tube for receiving gas through its lower end and having its gas inletting part connected to a filter aligned side-by-side at the upper portion of the spinning dope drop device, (iii) a spinning dope discharge tube extending from the lower portion of the spinning dope drop device and (iv) a hollow unit for dropping the spinning dope from the spinning dope inducing tube formed at the middle portion of the spinning dope drop device.        U.S. Pat. No. 6,989,125 (Boney, et al.) discloses a process of making a nonwoven web, resulting in continuous fiber nonwoven webs with high material formation uniformity and MD-to-CD balance of fiber directionality and material properties, as measured by a MD:CD tensile ratio of 1.2 or less, and laminates of the nonwoven webs. The invention also includes a method for forming the nonwoven webs, wherein a fiber production apparatus is oriented at an angle less than 90 degrees to the MD direction, and the fibers are subjected to deflection by a deflector oriented at an angle B, with respect to the centerline of the fiber production apparatus, where B is about 10 to about 80 degrees.        U.S. Pat. No. 4,233,014 (Kinney) discloses a process and apparatus for forming a non-woven web in which a bundle of untwisted filaments are charged upstream of a pair of elastomer covered counter rotating squeeze rolls and propelled through the nip of the rolls to a moving laydown belt, with the assistance of an electrostatic field developed between the rolls and the belt.        U.S. Pat. No. 6,616,435 (Lee, et al.) discloses an electrospinning method and apparatus for manufacturing a porous polymer web, which includes the steps of: forming, pressurizing and supplying at least one or more kinds of polymer materials in a liquid state; and discharging and piling the polymer materials to a collector through one or more charged nozzles, the collector being located under the nozzles and charged to have a polarity opposing the polarity of the charged nozzles, the collector moving at a prescribed speed.        U.S. Pat. No. 5,744,090 (Jones, et al.) discloses a process for the manufacture of conductive fibers, usable in electrostatic cleaning devices, in which the conductive fiber is formed from a mixture, including at least one fiber forming material and conductive magnetic materials, and the conductive magnetic materials are migrated toward the periphery of the fiber by application of a magnetic field to the fiber. The conductive fibers having the conductive magnetic materials located at the periphery of the fiber are preferably incorporated into an electrostatic cleaning device for use in an electrostatographic printing device.        U.S. Pat. No. 5,817,272 (Frey, et al.) discloses a process of making a biocompatible porous hollow fiber that is made of polyolefin material and is coated with a biocompatible carbon material is disclosed. The biocompatible hollow fiber produced can be used as exchange material, diaphragms and/or semipermeable membranes within devices, which will contact blood or plasma outside of the living body. The coated fiber is produced by introducing a preformed porous hollow fiber into an atmosphere of gaseous monomer vinylidene chloride and subsequent induction, e.g. by gamma radiation, of a graft-polymerization reaction to form a uniform polyvinylidene chloride layer. The ultimate coating is formed after a dehydrochlorination reaction in which hydrogen chloride is removed from the layer. The dechlorination reaction is typically performed by treating the fiber with hot concentrated aqueous ammonia solution. The reaction can be continued to reduce the chlorine content of the coating to less than 6% of its original value.        U.S. Pat. No. 6,858,168 (Vollrath, et al.) discloses an apparatus and method for forming a liquid spinning solution into a solid formed product, whereby the solution is passed through at least one tubular passage, having walls formed at least partly of semipermeable and/or porous material. The semipermeable and/or porous material allows certain parameters, such as the concentration of hydrogen ions, water, salts and low molecular weight, of the liquid spinning solution to be altered as the spinning solution passes through the tubular passage(s).        U.S. Pat. No. 6,444,151 (Nguyen, et al.) discloses an apparatus and process for spinning polymeric filaments, in which a melt spinning apparatus for spinning continuous polymeric filaments, includes a first stage gas inlet chamber adapted to be located below a spinneret and optionally a second stage gas inlet chamber located below the first stage gas inlet chamber. The gas inlet chambers supply gas to the filaments to control the temperature of the filaments. The melt spinning apparatus also includes a tube located below the second stage gas inlet chamber, for surrounding the filaments as they cool. The tube may include an interior wall having a converging section, optionally followed by a diverging section.        U.S. Pat. No. 6,110,590 (Zarkoob, et al.) discloses synthetically spun silk nanofibers and a process for making the same, in which a silk nanofiber composite network is produced by forming a solution of silk fiber and hexafluroisopropanol, wherein the step of forming is devoid of any acid treatment, where the silk solution has a concentration of about 0.2 to about 1.5 weight percent silk in hexafluroisopropanol, and where the silk is selected from Bombyx mori silk and Nephila clavipes silk; and electrospinning the solution, thereby forming a non-woven network of nanofibers having a diameter in the range from about 2 to about 2000 nanometers.        U.S. Pat. No. 6,265,466 (Glatkowski, et al.) discloses an electromagnetic shielding composite having nanotubes and a method of making the same. According to one embodiment, the composite for providing electromagnetic shielding includes a polymeric material and an effective amount of oriented nanotubes for EM shielding, the nanotubes being oriented when a shearing force is applied to the composite. According to another embodiment of the invention, the method for making an electromagnetic shielding includes the steps of (1) providing a polymer with an amount of nanotubes, and (2) imparting a shearing force to the polymer and nanotubes to orient the nanotubes.        U.S. Pat. No. 6,656,394 (Kelly) discloses a method and apparatus for high throughput generation of fibers by charge injection, in which a fiber is formed by providing a stream of a solidifiable fluid, injecting the stream with a net charge, so as to disrupt the stream and allowing the stream to solidify to form fibers.        U.S. Pat. Nos. 6,955,775 and 7,070,640 (Chung, et al.) disclose a process of making fine fiber material, including improved polymer materials and fine fiber materials, which can be made from the improved polymeric materials, in the form of microfiber and nanofiber structures. The microfiber and nanofiber structures can be used in a variety of useful applications including the formation of filter materials.        U.S. Pat. No. 6,753,454 (Smith, et al.) discloses electrospun fibers and an apparatus therefor. A fiber comprising a substantially homogeneous mixture of a hydrophilic polymer and a polymer, which is at least weakly hydrophobic is disclosed. The fiber optionally contains a pH adjusting compound. A method of making the fiber comprises electrospinning fibers of the substantially homogeneous polymer solution. A method of treating a wound or other area of a patient requiring protection from contamination comprises electrospinning the substantially homogeneous polymer solution to form a dressing. An apparatus for electrospinning a wound dressing is disclosed.        U.S. Pat. No. 5,911,930 (Kinlen, et al.) discloses solvent spinning of fibers containing an intrinsically conductive polymer, including a fiber containing an organic acid salt of an intrinsically conductive polymer distributed throughout a matrix polymer along, with a method for providing such fibers by spinning a solution, which includes an organic acid salt of an intrinsically conductive polymer, a matrix polymer, and a spinning solvent into a coagulation bath including a nonsolvent for both the organic acid salt of an intrinsically conductive polymer and the matrix polymer. The intrinsically conductive polymer-containing fibers typically have electrical conductivities below about 10.sup.−5 S/cm.        U.S. Pat. No. 6,695,992 (Reneker) discloses a process and apparatus for the production of nanofibers, including an apparatus for forming a non-woven mat of nanofibers, by using a pressurized gas stream, which includes parallel, spaced apart, first, second, and third members, each having a supply end and an opposing exit end. The second member is located apart from and adjacent to the first member. The exit end of the second member extends beyond the exit end of the first member. The first and second members define a first supply slit. The third member is located apart from and adjacent to the first member on the opposite side of the first member from the second member. The first and third members define a first gas slit, and the exit ends of the first, second and third members define a gas jet space. A method for forming a non-woven mat of nanofibers utilizes this nozzle.        U.S. Pat. No. 7,070,723 (Ruitenberg, et al.) discloses a method for spin-drawing of melt-spun yarns. A method is provided for simultaneous spin-drawing of continuous yarns consisting of one or more filaments, comprising the steps in which a melt of a thermoplastic material is fed to a spinning device, the melt is extruded through a spinneret, by means of extrusion openings with the formation of continuous yarns, the continuous yarns are cooled by feeding them through a first and a second cooling zone, wherein the continuous yarns are cooled essentially by a stream of air on passing through the first cooling zone and essentially by a fluid, consisting wholly or partly of a component that is liquid at room temperature, on passing through the second cooling zone, and the continuous yarns are then dried, subsequently drawn and wound up by means of winding devices, the method being distinguished in that the continuous yarns are fed through the first and second cooling zones at a speed of up to 500 m/min and that the residence time of the continuous yarns within the first cooling zone is at least 0.1 sec.        U.S. Pat. No. 7,105,058 (Sinyagin) discloses an apparatus and method for forming a microfiber coating, which includes directing a liquid solution toward a deposition surface. The apparatus includes a tube defining a volume through which the liquid solution travels. An electric field is applied between the origin of the liquid solution and the surface. A gas is injected into the tube to create a vortex flow within the tube. This vortex flow protects the deposition surface from entrainment of ambient air from the surrounding atmosphere.        U.S. Pat. No. 7,105,812 (Zhao, et al.) discloses a microfluidic chip with enhanced tip for stable electrospray ionization, in which a microfluidic chip is formed with multiple fluid channels terminating at a tapered electrospray ionization tip for mass spectrometric analysis. The fluid channels may be formed onto a channel plate that is in fluid communication with corresponding reservoirs. The electrospray tip can be formed along a defined distal portion of the channel plate that can include a single or multiple tapered surfaces. The fluid channels may terminate at an open-tip region of the electrospray tip. A covering plate may substantially enclose most portions of the fluid channels formed on the channel plate except for the open-tip region. Another aspect of the invention provides methods for conducting mass spectrometric analyses of multiple samples flowing through individual fluid channels in a single microfluidic chip that is formed with a tapered electrospray tip having an open-tip region.        U.S. Pat. No. 5,296,172 (Davis, et al.) discloses an electrostatic field enhancing process and apparatus for improved web pinning and uniformity in a fibrous web forming operation. The improvements are achieved by imposing an auxiliary electrostatic field above the fibrous web as it is pinned along a moving collection surface. An auxiliary electrostatic field enhancing plate is positioned above the web and collection surface and downstream of the laydown position where the web initially is deposited on the collection surface. The plate enhances the electrostatic field in the region above the collection surface and thereby increases the web pinning forces. When the invention is applied to a flash-spinning process, where trifluorochloromethane is used as the fluid medium, an auxiliary electrostatic field of between about 2 and 80 kV/cm, preferably between about 10 and 60 kV/cm, is applied by the plate.        U.S. Pat. No. 3,860,369 (Berthauer, et al.) and U.S. Pat. No. 3,851,023 (Berthauer, et al.) disclose apparatus for making non-woven fibrous sheet and a process for forming a web; U.S. Pat. No. 3,319,309 (Owens) discloses charged web collecting apparatus; and U.S. Pat. No. 3,689,608 (Hollbert, et al.) discloses a process for forming a nonwoven web.        U.S. Pat. No. 4,965,110 (Berry) and U.S. Pat. No. 5,024,789 (Berry) disclose a method and apparatus for manufacturing an electrostatically spun structure; U.S. Pat. No. 4,044,404 (Martin, et al.) discloses a fibrillar lining for a prosthetic device prepared by electrostatically spinning an organic material and collecting the spun fibers on a receiver; and U.S. Pat. No. 3,169,899 (Steuber) discloses non woven fibrous sheet of continuous strand material and the method of making same.        U.S. Pat. No. 7,105,124 (Choi) discloses a method, apparatus, and product for manufacturing nanofiber media; U.S. Pat. No. 7,081,622 (Kameoka, et al.) discloses an electrospray emitter for a microfluidic channel; U.S. Pat. No. 6,106,913 (Scardino, et al.) discloses fibrous structures containing nanofibrils and other textile fibers; U.S. Pat. No. 6,709,623 (Haynes, et al.) discloses a process of and apparatus for making a nonwoven web; and U.S. Pat. No. 6,790,528 (Wendroff, et al.) discloses production of polymer fibers having nanoscale morphologies.        Reneker, D. H., Yarin, A. L., Fong, H., and Koombhongse, S., “Bending instability of electrically charged liquid jets of polymer solutions in electrospinning,” Journal of Applied Physics, 2000, 87, No 9, pp. 4531-4547 discloses bending instability of electrically charged liquid jets of polymer solutions in electrospinning. Nanofibers of polymers were electrospun by creating an electrically charged jet of polymer solution at a pendent droplet. After the jet flowed away from the droplet in a nearly straight line, the jet bent into a complex path and other changes in shape occurred, during which electrical forces stretched and thinned it by very large ratios. After the solvent evaporated, birefringent nanofibers were left. The reasons for the instability are analyzed and explained, using a mathematical model. The rheological complexity of the polymer solution is included, which allows consideration of viscoelastic jets. It is shown that the longitudinal stress caused by the external electric field acting on the charge carried by the jet stabilized the straight jet for some distance. Then a lateral perturbation grew in response to the repulsive forces between adjacent elements of charge carried by the jet. The motion of segments of the jet grew rapidly into an electrically driven bending instability. The three-dimensional paths of continuous jets were calculated, both in the nearly straight region, where the instability grew slowly and in the region where the bending dominated the path of the jet. The mathematical model provides a reasonable representation of the experimental data, particularly of the jet paths determined from high speed videographic observations        Warner, S. B., Buer, A., Grimler, M., Ugbolue, S. C., Rutledge, G. C. and Shin, M. Y., “A Fundamental Investigation of the Formation and Properties of Electrospun Fibers”, National Textile Center Annual Report, 1998 discusses the fundamental engineering science and technology of electrostatic fiber production (“electrospinning”). Electrospinning and its capabilities for producing novel synthetic fibers of unusually small diameter and good mechanical performance (“nanofibers”), and fabrics with controllable pore structure and high surface area are discussed. The following items are included: design and construction of process equipment for controllable and reproducible electrospinning; clarification of the fundamental electrohydrodynamics of the electrospinning process and, correlation to the polymer fluid characteristics; characterization and evaluation of the fluid instabilities postulated to be crucial for producing ultrafine diameter fibers; characterization of the morphology and material properties of electrospun polymer fibers; development of techniques for generating oriented fibers and yarns by the electrospinning process; and productivity improvement of the electrospinning process.        
For the foregoing reasons, there is a need for apparatus and methods that control the jet and minimize instabilities of the jet as it travels from the nozzle to the collector plate. The apparatus and methods should be capable of controlling the jet, the path of the jet, and the concentration of solution during electrospinning.
The apparatus and methods should also be capable of controlling the jet and minimizing instabilities for fluids of different viscosities, and should be capable of controlling the jet, during the use of extreme temperatures and high strength electrical fields.
The apparatus and methods that control and minimize instabilities of the jet should be capable of improving efficiency, productivity, and economy of the electrospinning process. The apparatus and methods should also be capable of more accurate use of fluids, improvements in production and formation of fibers, and improvements in the production rate, fiber diameter distribution, measure, and characterization of the electrospun fiber properties in terms of size, orientation and mechanical properties.